Second-harmonic phase mismatch
= Second-harmonic phase mismatch
{title2=$\Delta k=k_{2\omega}-2k_\omega$}
The difference between the harmonic's <wave number> and that of the quadratic driving <electric polarization>. It has units of inverse <length>. For collinear modes, $\Delta k=2\omega(n_{2\omega}-n_\omega)/c$, so matching <frequencies> alone does not ensure matching spatial <wave phases>.